In Three-Dimensional High Efficiency Video Coding (3D-HEVC), which is a recent international three-dimensional video coding specification, an intra-frame skip mode is used as a coding technology for depth image coding. This technology adopts an intra-frame prediction method. That is, pixels in a current pixel block are predicted by using reconstructed values of other pixels at positions that are spatially adjacent to the current pixel block. Since the current pixels to be predicted and the adjacent pixels used in prediction are in a same frame, this method is referred to as intra-frame prediction or intra prediction. After the intra-frame prediction is performed, a difference between a predicted value of a pixel and an original value of the pixel is calculated, according to a conventional intra-frame coding mode, and difference information and prediction method information are encoded into a video bitstream and are then transmitted to a decoder. Accordingly, the decoder performs intra-frame prediction by using the same prediction method to obtain the predicted value of the pixel, and adds the difference to the predicted value, thereby obtaining the reconstructed value of the pixel. However, in the intra-frame skip mode, after the intra-frame prediction is performed, the difference information is not transmitted. That is, the predicted value of the pixel is regarded as the final reconstructed value of the pixel. Because difference compensation is not performed, the reconstructed value may have a specific distortion compared to the original value before encoding, but coding compression efficiency is improved because the difference information is not required to be transmitted. In practice, there are many flat regions in the depth image. In the flat regions, a difference between the predicted value of the pixel obtained by the intra prediction and the original value of the pixel is very small. Thus, even if the difference information is not transmitted, the distortion of the reconstructed value is not large. Thus, this technology is adopted in 3D-HEVC to code the depth image.
In 3D-HEVC, an inter prediction mode and other intra prediction modes are still maintained with respect to coding the depth image. Thus, in 3D-HEVC, a flag is fixedly transmitted in the bitstream so as to indicate whether the intra-frame skip mode is used to encode the current pixel block, and in this regard, the flag is transmitted prior to other pieces of information. Thus, when the flag indicates that the intra-frame skip mode is used for the current pixel block, only the intra-frame prediction method used in prediction is subsequently transmitted, and none of the other pieces of information is transmitted. When the flag indicates that the intra-frame skip mode is not used for the current pixel block, other pieces of information, e.g., a prediction mode used for the current pixel block, a block partitioning method, motion information, a prediction direction, residual information, etc., are subsequently transmitted.
The method of indicating the intra-frame skip mode in 3D-HEVC is simple, but efficiency is low. This is because the flag in this method is fixedly transmitted, regardless of additional information and a type of a data unit including the current pixel block. In practice, in the video coding specification, coding modes supporting coding of a pixel block included in different types of data units are different. For example, the encoder can only select the intra-frame coding mode for an intra prediction encoding frame, and thus, a probability of selecting the intra-frame skip mode is high. The encoder may select the intra-frame prediction mode or the inter-frame prediction mode for an inter prediction encoding frame, and thus, the probability of selecting the intra-frame skip mode is decreased accordingly. Thus, efficiency of fixedly transmitting a 1-bit flag is low. For example, when an occurrence probability of the intra-frame skip mode is very low, in order to indicate whether the intra-frame skip mode is used, the 1-bit flag still has to be transmitted.